Nowadays, the electric industry uses a variety of dielectric fluids, such as, mineral oils, petroleum derivatives, silicone fluids and synthetic hydrocarbon oils used which are used in transformers, conductive cables and capacitors. Examples of such fluids are described in U.S. Pat. Nos. 4,082,866, 4,206,066, 4,621,302 5,017,733, 5,250,750, and 5,336,847.
Such fluids show good dielectric characteristics, however they have important drawbacks with regard to ecological issues. The main disadvantage of such fluids is that due to its chemical composition (high molecular weight), they are not biodegradable. In recent years, the electrical industries face the challenge of complying with new environmental and governmental regulations, which demand to the industry to offer “green” products, that is, to offer products that are environmentally friendly. Said environmentally tendency has propitiated the necessity of modifying processes and changing product compounds in order to fulfill said new regulations and be able to offer ecological products.
In order to fulfill the above referred requirements, electrical industry has concentrated their efforts in the production of several kinds of dielectric fluids from vegetable oils obtained from oilseeds. Several seeds have been tested among which are the sunflower, rapeseed, linseed, soybean, cotton, safflower, corn and olive seeds. Examples of vegetable oils used as dielectric fluids are described in the following patents GB-609133, CA-2204273, U.S. Pat. Nos. 5,766,517, 5,949,017, 5,958,851, 6,037,537, 6,159,913, 6,184,459, 6,207,626, 6,245,726, 6,274,067, 6,280,659, 6,312,623, 6,340,658, 6,347,033, 6,352,655, 6,398,986, 6,485,659, 6,645,404, 726,857, 6,905,638, and 7,048,875 and in the following patent applications US-2002049145, US-2005040375, US-2006030499, WO-2007029724, and MX-PA06002862.
The idea of using vegetable oils obtained from oilseeds as isolating and cooling means in electric apparatuses is not entirely new. In the past, said oils were not considered adequate for using them as dielectric fluids, mainly due to the low resistance to oxidation of said oils compared to synthetic dielectric fluids. The nature of the compounds present in vegetable oils cause the oxidative reaction to accelerate in the presence of oxygen propitiating the polymerization process and as a result, a degradation of the fluid properties. Furthermore, said oils have some degree of electrical conductivity, which increases as the properties of the oils are degradated by oxidation, polymerization and hydrolysis reactions. The raise in the electrical conductivity is caused by the increase of polar compounds formed by said reactions.
For example, soybean oil (Glycine max) as an isolating and cooling medium for electric apparatuses has not been extensively used due to its low stability to oxidation caused by its high content of polyunsaturated fatty acids.
On the other hand, vegetable oils for human consumption do not have the necessary dielectric properties for its use in electric equipment as isolating and cooling medium due to its content of polar compounds which are not required to be eliminated for domestic use.
It is worthy of note that vegetable oils are mainly comprised by a natural mix of triglycerols also known as triglycerides. In addition to the content of triglycerides in vegetable oils there is a further content of other compounds such as tocopherols, sterols, and sterol esters as well as other compounds and impurities such as phosphatides, free fatty acids, chlorophylles, metallic traces, oxidation compounds, etc.
The triglycerides are the result of an esterification reaction between three fatty acids and glycerin. The acyl groups or triglycerol fatty acids can be similar or different, or each distinct from the other two. The fatty acids can be saturated when they have no double bonds, mono-unsaturated when they have a double bond and poly-unsaturated when they have two or more double bonds.
At present, it has been demonstrated that by modifying some oil production processes such as hydrogenation or by incorporating some antioxidants and/or synthetic additives capable of retarding, preventing or avoiding oxidation, it can be improved the vegetable oil stability to oxidation in order to make feasible the use of vegetable oils in electric equipment. However, the addition of antioxidants and/or synthetic additives reduces the ability of the vegetable oil to biodegrade, which is not convenient.
As it was previously described, the vegetable dielectric oils that have been produced, contain antioxidant and/or synthetic compounds in order to compensate its poor stability to oxidation. Furthermore, some compounds are also incorporated in order to improve its pour point, which comprises the lower temperature at which the oil is able to flow.
The above referred vegetable oils are considered biodegradable, however, the chemical composition of the antioxidants and/or additives incorporated, affect its biodegradation characteristics.
Similarly, it is known that several of the antioxidants and/or synthetic additives used nowadays, have toxic characteristics which are hazardous for the personnel that are in contact with the product as well as for the environment in case there is a spillover of the product. Examples of some of said compounds are: butylated hydroxyanisole (BHA) and butylhydroxytoluene (BHT) among others.
Among all vegetable oil components, the tocopherols are natural oxidants which are convenient to preserve in the oil, however there are some other compounds or impurities whose content must be lowered or eliminated from the oil in order to make it feasible for industrial applications, which may be achieved by submitting the oil to a process called refinement.
The oil refinement process is capable of eliminating more compounds and impurities than when the operation parameters are changed; therefore, the oil refinement process is the best way to improve the quality of edible vegetable oil, by removing a percentage of the compounds and impurities that are the cause of its low dielectric capacity without changing the fatty acids that are esterified to glycerin.
The oil refinement limitations have been gradually overcome as said process has been studied, thanks to which it has been possible to find which compounds act as pro-oxidants and which compounds and in which quantities act as antioxidant, thanks to which it is possible to adjust the oil refinement process variables in order to obtain a product having good stability to oxidation at the lowest cost.
Many known processes for obtaining similar fluids use as raw material, RBD oils (refined, bleached and deodorized) obtained by the RBD oil refinement which are submitted to several steps in order to obtain an adequate dielectric oil that can be used as isolating and cooling medium.
For example, U.S. Pat. Nos. 5,949,017, 6,274,067, 6,312,623, 6,645,404, and 7,048,875 describe vegetable oils having a high content of oleic acid and adequate dielectric properties and methods for obtaining said vegetable oils which are adequate for being used as isolating and cooling medium. The processes described in said patents uses as raw material RBD oils and submit them to an additional purification step somewhat similar to the bleaching step, in order to lower or eliminate the polar materials from the oil, which give the oil bad dielectric properties, and add antioxidants and/or synthetic additives to the oil in order to obtain a good stability to oxidation.
In view of the above referred limitations and requirements, it is evident the necessity of providing a high purity dielectric vegetable oil without any content of antioxidants and/or external additives, having good biodegradation properties, which can be obtained by a modified RBD process and which can comply with certain necessary physic properties in order to be considered as a dielectric fluid.
By using said modified RBD process it is possible to obtain a high purity dielectric vegetable oil at a minimum cost and without drastically changing the RBD process for obtaining edible oil actually in use.
Additionally the present invention also provides an electric apparatus using said high purity dielectric vegetable oil without any content of antioxidants and/or external additives.